In conventional “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and a lithographic ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The lithographic ink is eventually transferred to the surface of a material upon which the image is to be reproduced.
Lithographic printing plate precursor useful to prepare lithographic printing plates typically comprise one or more imageable layers applied over the hydrophilic surface of a substrate. The imageable layers include one or more radiation-sensitive components that can be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Following imaging, either the imaged (exposed) regions or the non-imaged (non-exposed) regions of the imageable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the imaged regions are removed, the precursor is considered as positive-working. Conversely, if the non-imaged regions are removed, the precursor is considered as negative-working. In each instance, the regions of the imageable layer (that is, the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel lithographic printing ink.
Direct digital imaging has become increasingly important in the printing industry. Imageable elements for the preparation of lithographic printing plates have been developed for use with infrared lasers that image in a platesetter in response to signals from a digital copy of the image in a computer. This “computer-to-plate” technology has generally replaced the former technology where masking films were used to image the elements.
In general, lithographic printing plate precursors contain a colorant (dye or pigment) in the radiation-sensitive composition that has the function of making the resulting image visible in order to be evaluated by optical density measurements before being mounted on a printing press. In other words, such colorants provide contrast between the image and the background. Certain lithographic printing plate precursors cannot contain a colorant for different reasons. For example, the imaged precursors that are usually developed on-press have a colorless coating because if a colorant is present, it could contaminate the lithographic printing ink and the fountain solution used for development and printing, with the result of altering the printed color shades. However, sometimes it is necessary for such lithographic printing plates to be used the same way as those developed off-press. In such instances, the image needs to be seen and evaluated.
Other lithographic printing plate precursors contain materials that are not compatible with contrast-providing colorants. The resulting lithographic printing plates have a faint colored image that is difficult to distinguish from the anodized aluminum substrate background. This low image contrast makes it almost impossible to evaluate these printing plates for image quality, such as image resolution as measured using optical density measurements before mounting the printing plates onto a press. Such “colorless” printing plates are also difficult to “register” (align) when mounting them onto a printing press. Image registration (alignment) is very important in color printing in order to ensure image sharpness (resolution) and correct tinting shades.
In addition, it is a well known phenomenon that offset lithographic printing plates containing sufficient coloration to be visible and measureable, when subject to a development stage, leave significant contaminants of the dye used to color the plates within the developer unit. Such coloration has been tolerated as an inevitable part of the process. Since any resinous material left within the developer unit has to be removed, dye residues color the developer and cannot be easily completely removed. Thus, there is a need to avoid this problem that occurs in development.
U.S. Pat. No. 6,451,491 (Dhillon et al.) describes high loading of contrast-providing pigments into the imaging layer using specific poly(vinyl acetal) polymers and specific combinations of loading solvent mixtures. Such high amounts of pigments can destabilize the imaging chemistry or the developers used to remove non-imaged regions in negative-working lithographic printing plate precursors.
Other contrast-providing colorants are obtained from leuco dyes that become colored in the presence of an acid or thermal acid generator, as described, for example in U.S. Pat. Nos. 7,402,374 (Oohashi et al.), 7,425,406 (Oshima et al.) and 7,462,440 (Yamasaki). These imaging materials have a number of disadvantages that provide reasons for not using them. In part, these leuco dyes adversely affect the shelf-life, the mechanical properties, and the run-length of the photosensitive coating.
U.S. Patent Publication 2010-0316956 (Memetea et al.) describes a method of preparing a lithographic printing plate by exposing a negative-working lithographic printing plate precursor having an optical density of OD1 and applying a coloring fluid to the exposed precursor such that the optical density in the exposed regions is OD2 that is greater than OD1. This coloring fluid comprises a water-insoluble colorant and a water-miscible solvent that is capable of swelling the exposed imageable layer. While this method provides a way to provide coloration in lithographic printing plates, there is a need to improve upon the method by further reducing residual dye contamination of the background, and further improving printing plate run length.
An alternative approach is to add a dye such as Crystal Violet to the processing liquid. While this approach provides the desired coloration, it can cause contamination in the processing bath.
U.S. Patent Publication 2009-0047599 (Home et al.) describes the use of spirolactams or spirolactones in the processing liquid. Such compounds are colorless in the liquid but become colored when the pH is decreased. While this also provides desired coloration, it can be difficult to regulate the pH so that the color change occurs only after processing and not in the processing liquid itself and that the changes do not affect the processing process and subsequent plate performance.
There is a need for an improved means for providing contrast between the image and background of lithographic printing plates, especially those prepared from negative-working lithographic printing plate precursors. It is also desirable that some embodiments of precursors are designed for on-press development.